It really depends on what engines you're using and what load you're applying to the engines.

anyone found a good config for all the gearboxs?

You don't need the clutch, since the game doesn't simulate gears of two different speeds grinding.

Basically, configuration can involve trial and error. Set all of them to a ridiculously high number, and then configure the first one to a reasonable number, test it, adjust it or move onto the next gear.

Place an RPS/torque meter on the output end of your gearboxes, and connect the RPS output to the RPS input on the controller.

With the somewhat recent update that completely changed the torque system, having more than a couple gearboxes can significantly reduce the power output.

Do you have a example how to use this in a setup?